An error checking procedure for the proper operation of disk drives, primarily magnetic disk drives, is to conduct a "time out test" at the start-up of the disk drive. In a time out test, if the disk drive does not reach full operational speed within the time out or specified period, it is deemed an error. Often, an error signal is sent by the drive controller to a host system and the drive is turned off.
However, both ball bearing and oil bearing spindles have increased drag torque at low temperatures, primarily due to increased viscosity of the oil. The increased drag is most significant for oil bearing spindles, which may be used in very high speed hard disk drives, such as drives that operate at 10,000 revolutions per minute (rpm). The increased drag torque is also prevalent in ball bearing spindles used in 3.5 inch form factor hard disk drives, but is greater on smaller drives where the bearing drag is a larger percentage of the total spindle drag compared to today's 3.5 inch drives.
The specific disadvantage is that the increased drag of the spindle bearing at low temperature start conditions may be so high that the spindle does not come up to speed in the specified time. For a 10,000 rpm, 1.6 inch high, 3.5 inch form factor oil bearing spindle, the difference in power due to additional bearing drag can be 1.0 Watt between 10 degrees C. and 25 degrees C. This corresponds to a significant difference in drag torque. To design the motor with a torque constant based upon this worst case start condition may significantly compromise the motor at nominal operating conditions. The increased drag at low temperature starts would demand a lower torque constant to meet necessary voltage headroom conditions for this worst case start condition. Specifically, increased torque requires increased current which in turn causes increased voltage drop across resistive elements in the driver circuit (including the motor stator windings). The voltage increase can be calculated from Ohm's law: V=I.times.R, where V is voltage, I is current and R is resistance. These higher voltages in the circuit take away from the voltage margin available for speed control (or voltage headroom). Since a large portion of the voltage in the circuit is due to the back EMF from the motor (which can be calculated as the motor torque constant in Nm/amp.times.the motor speed in radians/second), a typical design practice is to reduce the motor torque constant just enough so that at worst case drag conditions there is sufficient voltage headroom for adequate speed control.
The worst case drag condition exists at a cold temperature start. If this worst case low temperature start condition could be handled in some other way, the motor design would not have to be compromised with a reduced torque constant.
The spindle motor bearings in typical disk drive spindle motors are located very close to the motor windings. The motor windings are the most significant source of heat in a motor. Thus, as the motor starts, the bearings heat up and increase in temperature. The bearing drag torque is a function primarily of the viscosity of the bearing grease base oil and of the stiffness of the grease, both of which reduce with increased temperature. So the bearing drag torque reduces with time shortly after the motor starts spinning due to the heating of the bearings. By allowing more time to spin up to speed before closed loop motor control takes over (which necessitates voltage headroom) the bearing drag can be reduced which in turn will allow more voltage headroom.